The technical terms involved in this invention are described as follows:
Deposition: the technological process that the raw material of optical fiber sets off chemical reaction and generates doped silica glass in certain environment;
Collapsar: the technological process that the deposited hollow glass tube is gradually burned into solid glass rod under certain heat source;
Bushing: super pure silica glass tube with certain sectional area and dimensional homogeneity;
Parent tube: super pure silica glass tube for deposition;
Refractive index profile (RIP): relation curve between refractive index and radius of optical fiber or optical fiber preform (including optical fiber core rod);
Absolute refractive index difference (δn): the difference between refractive indexes of pure silica glass and parts of optical fiber preform;
Relative refractive index difference (Δ%):
            Δ      ⁢      %        =                                        n            i            2                    -                      n            0            2                                    2          ⁢                      n            i            2                              ×      100      ⁢      %        ,wherein, n1 is the refractive index of layer i fiber material, and n0 is the refractive index of pure silica glass.
RIP distribution parameter (α): n(r)=n1[1−2Δ(r/α)α]0.5, wherein, n(r) is refractive index at radius r, a is fiber (or core rod) radius, n1 is the max. refractive index in fiber (or core rod) core space, and α is RIP distribution parameter;
Effective area:
            A      eff        =          2      ⁢      π      ×                                    (                                          ∫                0                ∞                            ⁢                                                E                  2                                ⁢                r                ⁢                                                                  ⁢                                  ⅆ                  r                                                      )                    2                                      ∫            0            ∞                    ⁢                                    E              4                        ⁢            r            ⁢                                                  ⁢                          ⅆ              r                                            ,wherein, E is transmission-relating electric field, and r is fiber radius;
PCVD: plasma chemical vapor deposition.
Fiber laser is a kind of laser that adopts optical fiber as laser medium to acquire the laser output of corresponding waveband via the different rare earth ion doped in fiber ground substance material. Its applying field has been rapidly spreading from the current most matured fiber communication network to other more extensive laser applying fields, such as processing and treatment of metal and nonmetal materials, laser engraving, laser product marking, laser welding, welding seam cleaning, precision drilling, laser detection and measurement, laser graphic arts imaging, laser radar system, pollution control, sense technology, space technology and laser medicine, etc. For conventional single mode fiber laser, the pump list input in fiber core is also required to be single mode, which limits the pump light input efficiency, and leads to relatively low output power and efficiency of fiber laser. Doubly coated fiber provides an effective technological approach to improve output power and conversion efficiency of fiber laser, and changes the history that fiber laser can only be used as a photonic device with small power. Taking into consideration the factors of conversion efficiency, laser damage threshold and base loss, the rare earth doped silica doubly coated fiber is the optimal choice to realize high-efficiency fiber laser or amplifier.
With the rapid development of semiconductor laser pumping, laser coupling and other energy photoelectronic technologies, the multimode pumping technology has developed from the original end pump technology to current side pump technology, from single pump technology to multi pump set technology, accordingly, the power of fiber laser has also developed from the original milliwatt level to kilowatt level, and even myriawatt level. Ytterbium-doped, erbium-doped, ytterbium and erbium doped, thulium-doped, holmium-doped, neodymium-doped, praseodymium-doped, samarium-doped and other various new type high-power fiber lasers have been widely applied, and the output power of single doubly coated fiber laser has been gradually increasing. Currently, the continuous laser output power of single ytterbium-doped high-power fiber laser in foreign countries has reached 9.6 kW, while that in China is only 1.64 kW; the continuous output laser power of single thulium-doped high-power fiber laser in foreign countries has reached 1000 W, while that in China is only 100 W. These new type fiber lasers have the advantages of beam quality close to diffraction limit, long life (MTTF over 100,000 hours), high efficiency of photoelectric conversion, compact shape, low operating cost, easy maintenance and application, etc.
At present, the high-power fiber laser generally adopts conventional large-mode AOF, with the following technical difficulties, for example: to expand mode diameter is one of the main approaches to improve the loading power of fiber lasers, but the enlarged mode diameter will cause decrease of bean quality, increase of bending loss, and other negative effects; as to the conventional AOF, the mode diameter is enlarged to improve fiber loading power on one hand, the numerical aperture of optical fiber core has to be decreased to assure beam quality, both of which lead to technical difficulty, while cannot significantly improve the mode. In addition, when enlarging the mode diameter, the bending loss is rapidly increasing, which causes optical power leakage and even fiber damage, and finally abnormal operation. Moreover, as to the current large-mode AOF, the external coating adopts organic resin material of low refractive index, with low temperature resistant properties, while the optical fiber surface temperature is over 100° C. when high-power fiber laser is working at high power for long time, therefore, the damaged or carbonized external organic coating material causes AOF failure. These problems, as critical to improving the practical reliability of high-power fiber laser, must be solved as soon as possible.
The kind of large-mode optical fiber, disclosed by the Chinese patent of invention CN1667439 (application no. 200410011158.5 and disclosure date of 09-14-2005), adopts the fiber structure of asymmetrical multi-coated ring fiber core that is composed of multi arcs, with specified radius of circular arc and concave arc, complex construction, and difficult realizing technology, also, the mode of this large-mode fiber is expanded, and the bending loss is relatively excessive, therefore the beam quality is relatively poor.
The US patent US2006/0103919A1 describes a high-level large mode field active optical Fiber that adopts channel construction to filter the high-level mode in the diffused large-mode through leakage channel, which does not only expands the mode but also improves the output beam quality, at the cost of optical power, with low efficiency of optical utilization and vulnerable fiber; in addition, the optical fiber core described in this patent has deeper refractive index depression, which is the critical defect leading to hollow laser beam, and consequently poor output laser beam quality.
In summary, the above-mentioned patents have not effectively solved the technical difficulty to improve transmission power while remaining excellent beam quality for large-mode AOF, and have not mentioned solution to the technical problem of high-power outer lower coating damage.